Common Yellowthroat

This HY COYE in September has completed its preformative molt having replaced all lesser, median and greater coverts and the carpal covert, the molt limit between the outermost greater covert and innermost primary covert (larger red arrow).

Notice also the subtle difference between the replaced alula covert (A1) and main alula feather (A2) (smaller red arrow) and compare the main alula feathers (A2/A3) with the adult birds below. By spring the wear to the retained juvenal alula feathers will be more noticeable creating a more obvious molt limit between the, by then, warn and lightly pigmented lower alula feathers and replaced alula covert.

Molt limits like this can be quite subtle with birds in the hand and for this reason, especially after adults of these same species begin to show signs of approaching the end of their complete prebasic molt age should always be confirmed by additional criteria such as skull ossification.

This HY in August is in the midst of its preformative molt with median coverts and carpal covert in sheath.

Notice the buffy tips to the as yet unmolted median covert and greater coverts. Streaked/spotted plumage is a characteristic of juvenile plumage to provide camouflage for the period when a fledging leaves the nest and is vulnerable to predation. Buffy tips like these are often diagnostic of retained juvenal feathers in many species.

This HY COYE in September is also in the midst of its preformative molt and likely a late hatching bird. Notice again the median coverts and carpal covert in sheath and buffy tips to the outer greater coverts. Greater coverts molt proximally (towards the bird’s body) and this bird will likely retain these outer GCs forming a molt limit between GC6 and GC7 in sheath (red arrow).

Normally HY COYE replace all greater coverts in the preformative molt but if this bird was a late hatching bird as is likely, it may well not have the protein and energy resources to replace all of its GCs prior to migration which could explain the retained outer GCs shown here.

An after second year (ASY) male in May. The forehead, lores and auriculars are completely black.

The wing of the adult (ASY) male COYE above shows what definitive adult plumage looks like in the spring.

Notice the truncate shape of the visible remiges (primaries and secondaries) with very little wear, and the broad, greenish edging to the primary coverts and alula with no contrast between the alula covert (A1) and main lower alula feathers (A2 & A3).

Although the rectrices are showing some wear, consistent with Common Yellowthroats that spend their time low down in wet thickets both on their summer and winter grounds causing abrasion to feathers, these feathers are still showing truncate shape diagnostic of adults.

Adults in heavy prebasic molt like this bird can often still be identified as second year (SY) or after second year (ASY) until the alula feathers are lost. The alula covert (red arrow) is a key molt limit in many species. It is part of the tract of median coverts and molts with them prior to the molt of the main two lower alula feathers which are among the last feathers replaced.

As can be seen in this photo there is no contrast among the as-yet-unmolted alula feathers and the molted alula covert – both are worn but too brightly colored to be juvenal feathers. Within another week or so this bird would have to be aged simply after hatch year (AHY)

The rectrices of the same bird are as yet unmolted and show fairly heavy degree of wear but considering they belong to a COYE which spend most of their lives skulking low down in wet thickets and this bird has been through one long Neotropical migration and likely an entire breeding season in and out of the brush feeding hungry nestlings the degree of wear on these adult feathers was not nearly as much as it would have been on retained juvenal feathers and further supports that this bird is an adult alive in at least its third calendar year going through another definitive adult prebasic molt.

Post breeding adults like this AHY male in August are showing lots of wear after one long neotropical migration and having spent an entire breeding season in and out of brush feeding hungry nestlings.

The wing below is of the above AHY male at the start of its definitive prebasic molt with innermost primaries in sheath. During this molt all body and flight feathers will be replaced. 

The tail of the same bird showing extreme wear to the rectrices, particularly the central rectrices (r1) which attach above the plane of the remaining rectrices and serve as a protective covering for the feathers below. 

This AHY male is nearing the end of its definitive adult prebasic molt with the main alula feathers (A2/A3) in sheath and secondaries growing. In the adult PB molt the alula are molted during the molt of the primaries and are among the last feathers replaced – now that the old alula feathers have been replaced, unlike the example above this bird would be aged simply as AHY.

Birds in active molt are very helpful to banders learning ageing strategies using molt and plumage criteria. These ‘molt limits in the making’ allow us to see the sequence of events unfold as birds go through the molting process and to memorize the differences between retained and replaced feathers to help us when looking at these same species later in the fall and winter.

The wing of this after hatch year (AHY) male COYE shows the very end of this bird’s definitive adult prebasic molt when all body and flight feathers are replaced. Primaries molt distally (away from the birds body) with the outermost (P9 or P10) normally the last replaced. Secondaries molt proximally (towards the birds body) with the innermost (S6) normally the last flight feather replaced. In this photo P9 is still in sheath and S5 and S6 not quite fully grown (indicated with reds arrows).

Banders must be careful however with species like Common Yellowthroat which can often have very extensive 1st prebasic molts including some outer primaries and inner secondaries, and look closely at the rachis colour which is typically darker/blacker on molted feathers and lighter/browner on retained juvenal feathers. In this wing all of the remiges have been replaced and the rachises are all black and of the same diameter in each feather group.

The rectrices of the same bird show the rounded, more truncate shape typical of adult birds although when these feathers are very fresh they can show sharp tips to the very ends.

This adult (ASY) male COYE in April shows what definitive adult plumage looks like in the spring.

Notice how little wear there is on any of the flight feathers (primaries, secondaries and rectrices) and the truncate shape to these feathers. Notice also the broad, greenish edging to the primary coverts and alula and no contrast between the alula covert (A1) and main lower alula feathers.

This adult (ASY) male in April is showing more wear to the median and greater coverts but again notice the truncate shape of the visible primaries and secondaries, broad greenish edging to the primary coverts and alula and little contrast between the alula covert (A1) and adjacent lower alula feather (A2).

By the end of the breeding season adults show very worn feathers like this after hatch year (AHY) female COYE showing very worn flight feathers (primaries, secondaries and retrices)

Feather exposure to sun and abrasion is the major cause of feather deterioration. Exposure wears down feathers both through structural weakening and break-down as a result of solar irradiation, and through physical abrasion against foliage and the ground which is especially true of species such as Common Yellowthroats who spend much of their time skulking low down in wet thickets.